1. Field of the Invention
The invention is directed to a method of enhancing the rate of polymerization of nitroxyl radical-mediated polymerization of unsaturated monomers. More particularly, the invention is directed to the rate enhancement of radical polymerization in the presence of the 2,2,6,6-tetramethyl-1-piperidinoxyl radical (TEMPO).
2. Discussion of the Background
The concept of applying stable nitroxyl radicals to control free-radical polymerization was developed by Solomon and Rizzardo.sup.1 for a variety of monomers, but this original approach was limited to low molecular weight polymers. Subsequently, Georges et al adapted this approach to polymerization of styrene initiated by benzoyl peroxide in the presence of TEMPO.sup.2 yielding high molecular weight polymers with narrow polydispersities and molecular weights linearly increasing with conversion. Other groups have reported that well-defined polymers can also be obtained in thermal polymerization of styrene in the presence of TEMPO.sup.3 as well as in the polymerization of styrene initiated by various TEMPO adducts.sup.4,5 and other alkoxyamines.sup.6,7. Additionally, copolymers of styrene and other vinyl monomers have also been synthesized in a controlled manner by using this techniques.sup.8,9.
The control of the polymerization is achieved by reversible scavenging of the propagating radicals by a stable nitroxyl radical: ##STR2##
The above equilibrium is shifted strongly to the right side and thus the concentration of the active species is lower in the TEMPO-moderated polymerization than in a conventional free radical polymerization. Consequently, the rate of bimolecular termination, which is second order with respect to radical concentration, is lower and does not lead to a broadening of the molecular weight distributions, in contrast to conventional polymerizations. However, the rate of propagation (first order with respect to radical concentration) is also affected, although much less than the termination.
The long time required for completion of polymerization, even at relatively high temperatures and independently of the initiator concentration, is the most important disadvantage of this process. For example, bulk polymerization of styrene takes 20 hrs. to reach 60% conversion and 44 hrs. to reach final conversion at 120.degree. C. for a TEMPO adduct concentration of 0.01M. The reason for the slow polymerization rate is a low stationary concentration of the propagating radicals. At 120.degree. C., the equilibrium constant K=[P*].multidot.([T*]/[PT]), where P* is propagating radicals, T* is free TEMPO and PT is TEMPO-capped dormant chains, is on the order 10.sup.-11 mol.L.sup.-1. The experimentally determined stationary concentration TEMPO is on the order of 0.1% of the original concentration of TEMPO or its adduct.sup.11,12. Thus, the stationary concentration of propagating radicals is on the order of 10.sup.-8 mol.L.
The rate of the polymerization of styrene in the presence of TEMPO is independent of the concentration of the initiator and remarkably close to the rate of thermal polymerization under similar conditions. It has been shown that it is thermal self-initiation of styrene that provides a concentration of propagating radicals sufficient for obtaining reasonable, albeit slow, polymerization rates.sup.11. If the thermal initiation were absent, the polymerization would be even slower. Based on the above observations, a conclusion might be drawn that the rate of the TEMPO-mediated polymerization of styrene can be enhanced by decreasing the concentration of TEMPO and thereby, increasing the concentration of propagating radicals. Since the polymerization is controlled by the excess of TEMPO (a persistent radical effect.sup.14), this approach would undoubtedly lead to a higher rate of bimolecular termination and an increase of polydispersities if the concentration of TEMPO is brought too low and the rate of the trapping of the growing chains becomes lower than the rate of propagation. This might be avoided if the concentration of radicals is only few times higher than the concentration of radicals provided by thermal self-initiation, and the excess of TEMPO is continuously but slowly being removed from the system.
Georges et al reported that addition of a small amount of camphorsulfonic acid (CSA).sup.15 or 2-fluoro-1-methylpyridinium p-toluenesulfonate.sup.16 to a polymerization of styrene initiated by benzoyl peroxide in the presence of TEMPO increased the rate of polymerization significantly. But the addition of CSA or the salt also caused a broadening of polydispersities, particularly at the beginning of the polymerization. In the case of CSA as an additive, Georges et al postulated that the excess of TEMPO is removed by a rapid reaction with CSA.sup.17. This reaction irreversibly eliminates some of the nitroxide present at the beginning of the polymerization, thereby reducing the induction period. After the initial increase in the rate, the polymerization proceeds with virtually unchanged rate.sup.15. The mechanism of the rate enhancement in the presence of the salt is unknown so far.
A need continues to exist, therefore, for improved methods of controlling the rate of nitroxyl radical-mediated polymerization.